The invention relates to a comb filter for the rejection of intertrack chrominance crosstalk during reproduction of a colour video signal using the colour-under method described below.
In all currently commercially available home-entertainment video recorders the colour video signal is recorded in accordance with the colour-under method. In this method the luminance and the chrominance components are first extracted from the composite colour video input signal. In principle, the luminance component is frequency-modulated, high-pass filtered and applied to the first input of an adder stage. The chrominance component is down-converted to a frequency range situated below the frequency-modulated luminance component by means of a frequency converter, is low-pass filtered, and is applied to the second input of the adder stage, where it is combined with the frequency-modulated luminance component. The signal thus obtained is recorded in adjacent inclined tracks on a magnetic tape by means of rotating video heads.
During reproduction the signal is divided into the frequency-modulated luminance component and the down-converted chrominance component by means of a high-pass filter and a low-pass filter respectively. The luminance component is frequency-demodulated, low-pass filtered and applied to the first input of an adder stage. The chrominance component is back-converted, low-pass filtered, applied to the second input of the adder stage and combined with the frequency-demodulated luminance component.
In the above system disturbances in the colour reproduction occur because the reproducing heads not only read the recorded track but also pick up chrominance crosstalk components from adjacent tracks. Therefore, the known systems utilize a comb filter in the path of the reproduced chrominance component to remove crosstalk components originating from the back-converted chrominance component.
Such comb filters, which are known for example from the magazine "GRUNDIG TECHNISCHE INFORMATIONEN", 3-84, pp. 144-150, DE-PS 35 12 546, DE-PS 32 18 538, and EP-APPL. 0,183,438, comprise a first signal path for delaying the reproduced and back-converted chrominance component by a specific time interval one line period in the case of NTSC and two line periods in the case of PAL), a second signal path at whose output the reproduced, back-converted and non-delayed chrominance component is available, and an adder stage for adding the delayed and non-delayed chrominance components to each other.
This addition results in a chrominance signal freed from crosstalk components. Here, use is made of the fact that the colour information in successive lines changes only slightly. The colour information of adjacent lines can therefore be added without a visible deterioration in picture quality. However, in addition to the desired signal components the crosstalk components would then also be added to each other. This is precluded in that prior to recording appropriate phase shifts are applied to the chrominance components to ensure that during said addition the cross-talk components are in phase opposition and consequently cancel one another. How said phase shifts at the recording side are obtained in the case of a PAL signal recorded in accordance with the VHS standard is described in detail, for example, in the aforementioned magazine "GRUNDIG TECHNISCHE INFORMATIONEN". It is obvious that the phase reversals at the recording side must be cancelled during reproduction.
Moreover, DE-OS 3231 150 describes a PAL comb filter in whose first signal path the back-converted chrominance component is delayed by only one line period. In addition, this first signal path includes a PAL phase inverter, which may be of a controllable type, to ensure that, although in accordance with the PAL standard the periodicity of, the signal has a period corresponding to two line periods. The desired comb filtering is obtained, when the non-delayed chrominance component and the chrominance component having a delay of one line period are added.
From the aforementioned DE-PS 35 12 546 it is also known to connect a phase-shift circuit to the output of the delay circuit in the first signal path. The delay in this phase-shift circuit is adjustable and therefore any deviations from the nominal delay of the delay line can be compensated for.
In the majority of the currently commercially available home-entertainment video recorders the delay necessary for comb filtering is provided by a glass delay line. However, such a delay line has certain disadvantages such as undesired reflections, bulk, and the temperature-dependence of the glass.
CCD delay lines can also be used. In comparison with glass delay lines, CCD delay lines have the advantages of a substantially smaller size, an exact delay time, a non-critical adjustment, and the absence of undesired reflections. The delay time attainable by means of CCD delay lines depends on the number of "buckets" and the selected clock frequency.
However, it is found that the use of CCD delay lines in comb filters for filtering chrominance signals at their original frequency gives rise to problems both in production engineering and circuit design. For example, to date no satisfactorily performing CCD delay lines accommodated in one housing and having a delay corresponding to two line periods of the PAL signal are available. Moreover, the clock frequency necessary for the comb-filtering of chrominance signals at their original frequency (3.58 MHz for NTSC and 4.43 MHz for PAL) is situated in a more problematic higher frequency range.
This may necessitate, inter alia, intricate shielding measures. Another disadvantage is the higher power dissipation which is inevitable in the case of a higher clock frequency and a large number of "buckets". Moreover, a separate PLL-circuit is needed for generating the clock signal, because this signal must be phase-locked to the chrominance carrier frequency.